Getting to know a guy over facebook

Hello everyone, I've been wondering lately if there's any way of starting up a conversation with someone you like on facebook but have never really spoken to much in real life? There's this guy on my friends list who was the year above me in school, I added him back in 2010 because I liked him then and we had maybe one or two conversations in school and then he left to go to university and we've never seen each other since. Two guys have done this to me in the past (struck up random conversations over facebook when I'd never spoken to them before - again I knew them through school) and I remember finding it a bit odd at the start but not so much as the conversation continued. I considered poking him but quickly dismissed that as even weirder than randomly saying hi. So does anyone have any helpful advice? :)

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